Transport of Respiratory Gases In Lungs

Transport of Respiratory Gases

Transport Of Oxygen

Oxygen is transported from the alveoli to the tissue by blood in two forms:

1. As simple physical solution
2. In combination with hemoglobin.

As Oxygen  Simple Solution:

• Oxygen dissolves in the water of plasma and is transported in this physical form. The amount of oxygen transported in this way is very negligible.
• It is only 0.3 mL /100 mL of plasma. It is about 3% of the total oxygen in the blood. It is because of the poor solubility of oxygen in the water content of plasma.

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• Still, the transport of oxygen in this form becomes important during the conditions like muscular exercise to meet the excess demand of oxygen by the tissues.

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In Combination With Hemoglobin

Oxygen combines with hemoglobin in the blood and is transported as oxyhemoglobin.

• The transport of oxygen in this form is important because the maximum amount (97%) of oxygen is transported by this method.
• Oxygen combines with hemoglobin only as a physical combination. It is only oxygenation and not oxidation.
• This type of combination of oxygen with hemoglobin has got some advantages. Oxygen can be readily released from hemoglobin when it is needed.
• Hemoglobin accepts oxygen readily whenever the partial pressure of oxygen in the blood is more.
• Hemoglobin gives out oxygen whenever the partial pressure of oxygen in the bio is less.
• Oxygen combines with the iron in heme part of hemoglobin.
• Each molecule of hemoglobin contains 4 atoms of iron. The iron of the hemoglobin is present in a ferrous form.
• Each iron atom combines with one molecule of oxygen. After the combination, iron remains in ferrous form only.
• That is why the combination of oxygen with hemoglobin is called oxygenation and not oxidation.

Oxygen Carrying Capacity of Blood

The oxygen-carrying capacity of blood refers to the amount of oxygen transported by blood.

• One gram of hemoglobin carries 1.34 mL of oxygen. It is called the oxygen-carrying capacity of hemoglobin.
• The normal hemoglobin content in the blood is 15 g%. So, the blood with 15 g% of hemoglobin must carry 20.1 mL% of oxygen, i.e. 20.1 mL of oxygen in 100 mL of blood.
• But, the blood with 15 g% of hemoglobin carries only 19 mL% of oxygen, i.e. 19 mL of oxygen is carried by 100 mL of blood (Table 125-1).
• The oxygen-carrying capacity of blood is only 19 mL% because the hemoglobin is not fully saturated with oxygen. It is saturated only for about 95%.

Oxygen Hemoglobin Dissociation Curve

• The oxygen hemoglobin dissociation curve is the curve that demonstrates the relationship between the partial pressure of oxygen and the percentage saturation of hemoglobin with oxygen.
• It explains hemoglobin’s affinity for oxygen. Normally in the blood, hemoglobin is saturated with oxygen only up to 95%.
• The saturation of hemoglobin with oxygen depends upon the partial pressure of oxygen.
• When the partial pressure of oxygen is more, hemoglobin accepts oxygen and when the partial pressure of oxygen is less, hemoglobin releases oxygen.
• Method to Plot Oxygen Hemoglobin Dissociation Curve: Ten flasks or tonometers are taken. Each one is filled with a known quantity of blood with known concentration of hemoglobin.
• The blood in each tonometer is exposed to oxygen at different partial pressures.
• The tonometer is rotated at a constant temperature till the blood takes as much oxygen as it can.
• Then, the blood is analyzed to measure the percentage saturation of hemoglobin, with oxygen.
• The partial pressure of oxygen and saturation of hemoglobin are plotted to obtain the oxygen-hemoglobin dissociation curve.

Normal Oxygen Hemoglobin Dissociation Curve

• Under normal conditions, the oxygen hemoglobin dissociation curve is ‘S-shaped or sigmoid shaped.
• The lower part of the curve indicates the dissociation of oxygen from hemoglobin.
• The upper part of the curve indicates the acceptance of oxygen by hemoglobin depending upon the partial pressure of oxygen.

Normal Oxygen Hemoglobin P50

• P50 is the partial pressure of oxygen at which hemoglobin saturation with oxygen is 50%.
• When the partial pressure of oxygen is 25-27 mm Hg, the hemoglobin is saturated to about 50%.
• That is, the blood contains 50% of oxygen. At 40 mm Hg of partial pressure of oxygen, the saturation is 75%. It becomes 95% when the partial pressure of oxygen is 100 mm Hg.

Factors Affecting Oxygen Hemoglobin Dissociation Curve

The oxygen hemoglobin dissociation curve is shifted to the left or right by various factors:

1. Shift to the left indicates acceptance (association) of oxygen by hemoglobin
2. A shift to the right indicates the dissociation of oxygen from hemoglobin.

1. Shift to right:

1. The oxygen hemoglobin dissociation curve is shifted to the right in the following conditions:
2. Decrease in partial pressure of oxygen
3. Increase in partial pressure of carbon dioxide (Bohr’s effect)
4. Increase in hydrogen ion concentration and decrease in pH (acidity)
5. Increased body temperature
6. Excess of 2,3-diphosphoglycerate (DPG). It is also called 2,3-biphosphoglycerate (BPG).
7. DPG is a byproduct in the Embden-Meyerhof pathway of carbohydrate metabolism.
8. It is present in red blood corpuscles. It combines with (3 chains of hemoglobin. In conditions like muscular exercise and in high altitudes, the DPG increases in red blood corpuscles.
9. So, the oxygen-hemoglobin dissociation curve shifts to the right to a great extent.

2. Shift to left:

1. Shift of the oxygen hemoglobin dissociation curve to the left occurs in the following conditions:
2. In fetal blood: Because fetal hemoglobin has got more affinity for oxygen than adult hemoglobin.
3. Decrease in hydrogen ion concentration and increase in pH (alkalinity).

Bohr’s Effect

• Bohr’s effect is the effect by which the presence of carbon dioxide decreases the affinity of hemoglobin for oxygen.
• It was postulated by Christian Bohr in 1904. In the tissues, due to continuous metabolic activities, the partial pressure of carbon dioxide is very high and the partial pressure of oxygen is low.
• Due to the pressure gradient, carbon dioxide enters the blood, and oxygen is released from the blood to the tissues.
• The presence of carbon dioxide decreases the affinity of hemoglobin for oxygen.
• It enhances further release of oxygen to the tissues and the oxygen dissociation curve is shifted to the right. It is due to Bohr’s effect.

Factors influencing Bohr’s effect: All the factors, which shift the oxygen dissociation curve to the right (mentioned above) enhance Bohr’s effect.

Transport Of Carbon Dioxide

• Carbon dioxide is transported by the blood from tissues to the alveoli. The partial pressure and content of carbon dioxide in arterial blood and venous blood are given in Table.

Carbon dioxide is transported in the blood in four ways:

1. As dissolved form – 7%
2. As carbonic acid – negligible
3. As bicarbonates – 63%
4. As carbamino compounds – 30%.

Transport Of Carbon Dioxide As Dissolved Form

• Carbon dioxide diffuses into the blood and dissolves in the fluid of plasma forming a simple solution.
• Only about 3 mL/100 mL of plasma of carbon dioxide is transported as a dissolved state. It is about 7% of the total carbon dioxide in the blood.

Transport Of Carbon Dioxide As Carbonic

• ACIDPart of dissolved carbon dioxide in plasma combines with water to form carbonic acid.
• Though carbon dioxide is transported in this form, this reaction is very slow and it is negligible.

Transport Of Carbon Dioxide As Bicarbonate

• About 63% of carbon dioxide is transported as bicarbonate. From plasma, the carbon dioxide enters the RBCs.
• In the RBCs, carbon dioxide combines with water to form carbonic acid. The reaction inside RBCs is very rapid.
• The rapid formation of carbonic acid inside the RBCs is due to the presence of an enzyme called carbonic anhydrase. This enzyme accelerates the reaction.
• Carbonic anhydrase is present only inside the RBCs and not in the plasma. That is why the carbonic acid formation is at least 200-300 times more in the RBCs than in plasma.
• Carbonic acid is very unstable. Almost all carbonic acid (99.9%) formed in red blood corpuscles, dissociates into bicarbonate and hydrogen ions.
• The concentration of bicarbonate ions in the cell increases more and more. The increased concentration of bicarbonate inside the RBC causes the diffusion of bicarbonate ions through the cell membrane into the plasma.

Chiotide Shift or Hamburger Phenomenon

• Chiotide shift or Hamburger phenomenon is the exchange of a chloride ion for a bicarbonate ion across the erythrocyte membrane. It was discovered by Hartog Jakob Hamburger in 1892.
• Chloride shift occurs when carbon dioxide enters the blood from tissues. In plasma, plenty of sodium chloride is present. It dissociates into sodium and chloride ions.
• When the negatively charged bicarbonate ions move out of RBC into the plasma, the negatively charged chloride ions move into the RBC in order to maintain the electrolyte equilibrium (ionic balance).
• Band 3 proteins which act like antiport pumps in the RBC membrane are responsible for the exchange of bicarbonate ions and chloride ions.
• The bicarbonate ions combine with sodium ions in the plasma and form sodium bicarbonate. In this form, it is transported in the blood.
• The hydrogen ions dissociated from carbonic acid are buffered by hemoglobin inside the cell.

Reverse Chloride Shift

• Reverse chloride shift is the process by which the chloride ions are moved back into plasma from the RBC shift.
• It occurs when blood reaches the lungs and flows through pulmonary capillaries. It helps in the elimination of carbon dioxide from the blood.
• The bicarbonate is converted back into carbon dioxide, which has to be expelled. It takes place by the following mechanism:
• When the blood reaches the alveoli, sodium bicarbonate in the plasma dissociates into the sodium and bicarbonate ions. Bicarbonate ion moves into the RBC.
• It makes chloride ions move out of the RBC into the plasma, where it combines with sodium and forms sodium chloride.
• At the same time, oxygen also enters the RBC. It displaces hydrogen ions from hemoglobin.
• The hydrogen ion combines with the bicarbonate ion and forms carbonic acid, which dissociates into water and carbon dioxide. The carbon dioxide is expelled.

Transport Of Carbon Dioxide As Carbamino Compounds

About 30% of carbon dioxide is transported as carbamino compounds.

• Carbon dioxide is transported in the blood in combination with hemoglobin and plasma proteins.
• Carbon dioxide combines with hemoglobin to form carbamino hemoglobin or carbhemoglobin. And, it combines with plasma proteins to form carbamino proteins.
• The carbamino hemoglobin and carbamino proteins are together called carbamino compounds.
• The carbon dioxide combines with proteins or hemoglobin with a loose bond so that, carbon dioxide is easily released into alveoli, where the partial pressure of carbon dioxide is low.
• Thus, the combination of carbon dioxide with proteins and hemoglobin is a reversible one.
• The amount of carbon dioxide transported in combination with plasma proteins is very compared to the amount transported in combination with hemoglobin.
• It is because the quantity of proteins if? plasma is only half of the quantity of hemoglobin.

Carbon Dioxide Dissociation Curve

Carbon dioxide is transported in the blood as a physical solution and in combination with water, plasma proteins, and hemoglobin.

• The amount of carbon dioxide combining with blood depends upon the partial pressure of carbon dioxide.
• The carbon dioxide dissociation curve is the curve that demonstrates the relationship between the partial pressure of carbon dioxide and the quantity of carbon dioxide that combines with blood.
• Normal Carbon Dioxide Dissociation Curve
  The normal carbon dioxide dissociation curve shows that the carbon dioxide content in the blood is 48 mL% when the partial pressure of carbon dioxide is 40 mm Hg and, it is 52 mL% when the partial pressure of carbon dioxide is 48 mm Hg.
• The carbon dioxide content becomes 70 ml_% when the partial pressure is about 100 mm Hg.

Haldane Effect

• The Haldane effect is the effect by which the combination of oxygen with hemoglobin displaces carbon dioxide from hemoglobin.
• It was first described by John Scott Haldane in 1860. The excess oxygen content in the blood causes a shift of the carbon dioxide dissociation curve to the right.

Causes for Haldane effect

Due to the combination with oxygen, the hemoglobin becomes strongly acidic.

• It causes the displacement of carbon dioxide from hemoglobin in two ways.
• The highly acidic hemoglobin has a low tendency to combine with carbon dioxide.
• So, carbon dioxide is displaced from the blood.
• Because of the acidity, hydrogen ions are released in excess.
• The hydrogen ions bind with bicarbonate ions to form carbonic acid.
• Carbonic acid in turn dissociates into water and carbon dioxide.
• The carbon dioxide is released from the blood into the alveoli.

Significance of the Haldane effect:

Haldane’s effect is essential for:

1. The release of carbon dioxide from the blood into the alveoli of the lungs
2. Uptake of oxygen by the blood.